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Cell Biology & Energy (BIO 101) · Module 7 of 8

Photosynthesis: The Calvin Cycle & Adaptations

How captured energy builds sugar from CO2, and how some plants avoid wasting it in hot, dry climates.

In this module
  1. Carbon Fixation
  2. Reduction & Regeneration
  3. C3, C4, and CAM Adaptations
  4. Photorespiration
  5. Check yourself

Carbon Fixation

  • The Calvin cycle occurs in the stroma and begins with carbon fixation: the enzyme rubisco attaches CO2 to a 5-carbon molecule (RuBP), forming an unstable 6-carbon compound that splits into two 3-carbon molecules.
  • Rubisco is considered the most abundant enzyme on Earth, given how essential and widespread carbon fixation is.
Worked example

What is the role of rubisco in the Calvin cycle?

Solution. It catalyzes carbon fixation — attaching atmospheric CO2 to the 5-carbon molecule RuBP, the first step in converting inorganic carbon into organic sugar.

Reduction & Regeneration

  • ATP and NADPH from the light reactions convert the 3-carbon molecules into G3P (glyceraldehyde-3-phosphate); some G3P exits to build glucose and other organic molecules, while the rest regenerates RuBP to keep the cycle going.
  • It takes three turns of the Calvin cycle (fixing 3 CO2) to produce one net G3P available for making sugars, since most of the G3P produced must be recycled to regenerate RuBP.
Worked example

Why does it take three "turns" of the Calvin cycle to yield one net G3P for making glucose?

Solution. The Calvin cycle uses most of the G3P it produces to regenerate RuBP so the cycle can keep running; it takes three fixed CO2 molecules (three turns) to generate one G3P molecule actually available to build sugars.

C3, C4, and CAM Adaptations

  • C3 plants (most plants) fix carbon directly via rubisco, but rubisco can mistakenly bind O2 instead of CO2 in hot, dry conditions (photorespiration), wasting energy.
  • C4 plants (e.g. corn) spatially separate initial carbon fixation from the Calvin cycle, concentrating CO2 to minimize photorespiration; CAM plants (e.g. cacti) separate these steps by time, fixing carbon at night when stomata are open to reduce water loss.
Worked example

Why might a C4 or CAM adaptation benefit a plant living in a hot, dry climate?

Solution. Both adaptations minimize photorespiration and water loss — C4 by concentrating CO2 near rubisco, and CAM by fixing carbon at night when stomata can open without losing as much water to evaporation in the heat.

Photorespiration

  • Photorespiration occurs when rubisco binds O2 instead of CO2, especially when stomata close in hot, dry weather to conserve water — which also traps O2 and depletes CO2 inside the leaf — consuming energy without producing sugar.
Worked example

Under what conditions does photorespiration become more likely to occur?

Solution. In hot, dry weather, when a plant closes its stomata to conserve water, which also traps oxygen and depletes carbon dioxide inside the leaf, making it more likely rubisco binds O2 instead of CO2.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What enzyme catalyzes carbon fixation in the Calvin cycle?

2. What two molecules from the light reactions power the Calvin cycle's reduction step?

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